Designated Evil is when a "good guy" performs an act that's evil in and of itself but was necessary or pragmatic within the circumstances given, yet the author portrays it as unambiguously evil, possibly even a crossing of the Moral Event Horizon or at the very least a slide toward the dark side. The author doesn't bother to sufficiently elaborate on any other viable options the hero could have taken to deal with the problem faced; they just decide that the hero's actions were wrong.